1. Field of the Invention
The present invention relates to an image scanning/recording apparatus such as a composing scanner for separately scanning a plurality of originals to record pictorial images in which duplicate pictorial images are in countact with each other. More specifically, it relates to a method of and an apparatus for generating continuous halftone dots in connected portions of separate images for halftone processing, for use in such an image scanning/recording device.
2. Description of the Prior Art
Development of a plain layout scanner saving image memory has enabled low-cost implementation of a layout function for recording a plurality of pictorial images on photosensitive material along desired layout. In order to prevent displacement in registering etc. of resulting printed material thereby to improve the quality of the same, positional relation between respective recorded images must be correctly matched with each other between respective ones of four color printing plates. Therefore, in an image scanning method as disclosed in Japanese Patent Laying-Open Gazette No. 31359/1985 in the name of DAINIPPON SCREEN MFG. CO.,LTD., sub scanning positions theoretically obtained from the number of main scanning lines and main scanning positions are maintained in correspondence with actual sub scanning positions thereby to improve accuracy of registration between the respective recorded images. According to this method, a recording head can be maintained to be always in the same scanning locus in case of separately scanning a plurality of originals to record respective images on the photosensitive material, whereby scanning lines on the respective recorded images by separate scanning are matched with each other (i.e., scanning lines by separate scanning are connected in line), and hence continuity of the scanning lines between the respective recorded images can be maintained to avoid appearance of clearances and overlapping in boundaries between a plurality of duplicate recorded images.
In such a method, however, improvement of the picture quality is not necessarily sufficient with respect to recording of halftone images through a halftone dot generator. Assuming that, for example, a halftone repeating pattern is partially formed by one main scanning in halftone processing by a halftone dot generator, it is anticipated that such a situation as shown in FIG. 1(a) may be frequently caused that a recording region 31 for first scanning is completed in the middle of the pattern at a boundary X and a subsequent recording region 32 for second scanning is started from the middle of the pattern at the boundary X. If the second sub scanning is started from a position corresponding to the boundary X in this case, it is indefinite from which part of the halftone pattern the halftone dot generator genrates halftone dots (i.e., it is uncertain whether or not the halftone dot generator generates the halftone dots continuously to those in the first scanning), and hence the halftone pattern is made discontinuous along the boundary X as shown in FIG. 1(b) in most of the case. In broad perspective, discontinuous portions of haltone thus formed are visible as black creases when area mean density is increased in the vicinity of boundaries while the same are visible as white creases when the area mean density is reduced. In order to avoid such a problem, the second sub scanning may be started again from the scanning origin to form a haltone pattern which is similar to (overlapping with) the first pattern absolutely from the beginning, whereas, in this case, useless scanning is performed without recording until it reaches an actually necessary scanning point (i.e., a sub scanning position corresponding to the boundary X ) for a relatively long time, and hence the processing time is increased.